The present invention relates to bipolar transistors and, in particular, to a bipolar transistor that includes a compound semiconductor of a group III element and nitrogen (N) as a material.
As a type of bipolar transistor that includes a compound semiconductor of a group III element and nitrogen (N) as a material, there is known an HBT (heterojunction bipolar transistor) made of a gallium nitride/aluminum gallium nitride (GaN/AlGaN) material (refer to JP 2002-368005 A).
The HBT is provided with an n+-gallium nitride (GaN) subcollector layer (thickness: ˜1000 nm, silicon (Si) concentration as an n-type dopant: ˜6×1018 cm−3), an n-GaN collector layer formed in a central region of the upper surface of this subcollector layer, a base layer made of an AlGaN/GaN alternating layer (superlattice) (total thickness is 150 nm, thickness of each GaN layer is ˜3 nm, being undoped, and thickness of each AlGaN layer is ˜1 nm, concentration of Mg as p-type dopant therein being ˜1×1019 cm−3) and an AlGaN emitter layer (thickness: 150 nm, concentration of Si as n-type impurity: ˜6×1018 cm−3) formed in a central region of the upper surface of this base layer on the upper surface of a semi-insulative substrate made of, for example, sapphire or silicon carbide (SiC) A collector electrode is formed in each of regions of the upper surface of the subcollector layer, which correspond to both sides of the collector layer. A base electrode is formed in each of regions of the upper surface of the base layer, which correspond to both sides of the emitter layer. An emitter electrode is formed on the emitter layer.
As mentioned in the above prior art example, with regard to the dopants normally used in the III-N compound semiconductor, the n-type dopant is Si, and the p-type dopant is Mg. However, in the III-N compound semiconductor, the activation rate of Mg is low, and the mobility of holes is low. Therefore, the sheet resistance of the p-type base layer becomes high.
For example, in the case of a uniform base layer made of GaN, even if the acceptor concentration of Mg as the p-type dopant is set up to 5×1019 cm−3, there is caused a hole concentration of only about 8×1017 cm−3. Therefore, the sheet resistance of the base layer comes to have a high value of 100 kΩ/□. The same thing can be said for the superlattice base layer in the HBT of JP 2002-368005 A.
Therefore, the conventional HBT has a problem that the base resistance rb becomes high, and the power gain is low in the high-frequency region.
A unilateral power gain (represented by a symbol U(f)) at a frequency f in the high-frequency region of the HBT that has the uniform base layer is expressed by the following equation (1):U(f)=fT/(8πrbCcf2)  (1)where fT represents the current gain cutoff frequency, rb represents the base resistance value and Cc represents the collector capacitance.